Means for generating narrow microwave pulses

ABSTRACT

Described is a system for obtaining high power extremely short and narrow pulses in the neighborhood of nanoseconds, wherein the pulses are derived from longer pulses and more specifically by means of two sequential discharges in a non-resonant waveguide section. The narrow rectangular pulses are derived from a longer pulse which is caused to break down sequentially in two sections of a transmission line, each of which includes a low pressure gas acted upon by relatively intense applied fields. By adjusting the triggers for the respective gas discharges high power pulses of selectively variable width may be produced. The two low pressure arc discharges make possible the generation of extremely narrow microwave pulses without the microwave power initiating the breakdown.

United States Patent Goldie 1 Aug. 29, 1972 MEANS FOR GENERATING NARROWMICROWAVE PULSES Goldie, A Multikilowatt X-Band Nanosecond Source IEEETransactions on Microwave Theory and Techniques, Vol. MTT-l 5, Dec.1967, pp. 722- 731 Primary ExaminerRay Lake Assistant Examiner-SiegfriedH. Gn'mm Attorney-F. l-l. Henson, E. P. Klipfel and J. L. Wiegreffe [57] ABSTRACT Described is a system for obtaining high power extremelyshort and narrow pulses in the neighborhood of nanoseconds, wherein thepulses are derived from longer pulses and more specifically by means oftwo sequential discharges in a non-resonant waveguide section. Thenarrow rectangular pulses are derived from a longer pulse which iscaused to break down sequentially in two sections of a transmissionline,

each of which includes a low pressure gas acted upon by relativelyintense applied fields. By adjusting the triggers for the respective gasdischarges high power pulses of selectively variable width may beproduced. The two low pressure arc discharges make possible thegeneration of extremely narrow microwave pulses without the microwavepower initiating the breakdown. I

11 Claims, 5 Drawing Figures PULSE GENERATOR 29 16a -18 DELAY /21 12CIRCUIT 23 PWS 2 22 PWS l Patented Aug. 29, 1972 3,688,214

3 Sheets-Sheet 1 NO. 4 /PWS3 N0.2 17 DELAY CIRCUIT 16 TRIGGER DELAYPULSE CIRCUIT GENERATOR 29 DELAY 21 12 CIRCUIT 23/ LPws 2 W FIG 1 22pwsli' TR RECEIVER N0.3 w DELAY CIRCUIT RE-Y j; u 381 FIG. 2 M TRIGGER lPULSE 6a KPWS 43 GENERATOR LA 42 INVENTOR WITNESSES CUE 5&(

I W l 2 HARRY GOLDIE BYQZW a %%2avzz I 4 ATTORNEY Patented Aug. 29, 19723 Sheets-Sheet 2 31 BEAM CURRENT PULSES IN PWS 1, 2&3 0 1 LATEAFTERGLOW, (B) PWS 1, 2& 3 IEE Q( J I II J1; t I t DENSITY 1 IGI PBKDN III I ll t FIRST (E) DISCHARGE P3 H PWS 1& 2 t 'PMAXQPBKDN DELAY CIRCUIT59\59a/\ CIRCUIT TRIGGER PULSE M H6 4 CIRCUIT 64 Patented Aug. 29, 19723,688,214

5 Sheets-Sheet 5 [BEAM CURRENT PULSES IN PWS 63, 66, & 72 (A) I (AMPS)EARLY LATE 7; [AFTERGLOW AFTERGLOW (e/cc) MAX v i* T 1 PMAXT a (m k 1PLMAX (E) 0 t -1 2| "PMAXRI b2 (F) PR PULSE-TIME DIAGRAM FIG. 5

MEANS FOR GENERATING NARROW MICROWAVE PULSES BACKGROUND OF THE INVENTIONby selectively controlling the sequential breakdown in two respectivesections of a transmission line, which sections contain a low pressuregas enclosed by a pair of windows acting only as pressure barriers. Eachof the low pressure gas-filled sections are simultaneously acted upon bya relatively intense applied field.

In applicants US. Pat. No. 3,323,003, there is described and claimed athyratron waveguide switch which is an essential component of thepresent invention. In brief, the thyratron plasma waveguide switch,herein otherwise designated as a PWS, is an externally controlledgas-type switch for use in high speed, high power microwave applicationswhere extremely low loss and wide bandwidths are required. The device isnot RF- activated and therefore a source of triggering pulses isnecessary to actuate it. The electrical characteristics are such that itis relatively insensitive to ambient temperature variations.

In the PWS switch the control electrode is a section of microwave guidewhich is adapted to be inserted in the usual waveguide transmission linein which it is desired to control the propagation of microwave energy.The section of microwave is sealed to the envelope that encloses theanode and cathode and the waveguide section is provided with pressurewindows to complete the envelope that retains the hydrogen gasatmosphere around the electrodes.

When triggering pulses are applied between the control electrode and thecathode a plasma is generated in the region of the cathode and thiscauses the tube to discharge and a current arc to form between thecathode and the anode. This are creates a high density plasma thatextends across the section of the waveguide serving as the controlelectrode and this plasma serves as an RF barrier to provide attenuationto microwave energy.

In the previously mentioned patent a single PWS is used. In the presentapplication two PWS switches are used, one in each of the two respectivewaveguide sections which are coupled to two respective arms of acirculator through which the microwave energy is transmitted to theantenna.

In IEEE Transactions on Microwave Theory and Techniques, Vol. MT-lS, No.12, Dec. 1967, on pages 722 to 730, inclusive, an article by applicantentitled, A Multikilowatt X-Band Nanoseconds Source describes a systemwhich utilizes two PWSs in the two respective sealed waveguide sectionsconnected to the arms of the circulator between the source of microwaveenergy and the antenna. This article describes basically a system towhich the present invention is directed, namely, a system for generatingvery high power narrow rectangular pulses which are derived from longerpulses and utilizing the sequential breakdown in the two sections of atransmission line each of which contains a low pressure gas acted uponby relatively intense electric fields. The system described in thisarticle is an improvement over the system described in applicantsaforesaid patent. On the other hand, the present invention is animprovement over that system described in the published article.

SUMMARY OF THE INVENTION The present invention is predicated upon theconcept of priming a volume of low pressure gas with an electron cloudwhich is then allowed to cool to a Maxwellian velocity distributionclose to room temperature throughout the volume, followed by an RF pulsewhich causes an arc breakdown simultaneously throughout the gas volumeat a very fast rate to produce a very steep leading edge on the pulse.The initial cloud of electrons are formed by prefiring of the PWSsbefore the initiation of the microwave pulse which initiates andsupplies the energy for the ensuing discharge. If the microwaveconductivity is sufiiciently high the transmitted pulse will rapidlyfall to zero. Suitable timing circuits trigger the PWS switches and themicrowave generator.

The present invention expands the basic circuitry described in thearticle to use an output pulse of increased amplitude without change inthe pulse duration. The increase to the extent of potentially doublingof the output is obtained by the inclusion of a 3 dB power dividerconnected to the arm of the circulator. Since arm 1 of the circulatorhaving incident microwave power and arm 2 secs P/2, where P is theamplitude, the power of the incident power is essentially doubled,creating breakdown and this power is reflected into the load which isthe antenna.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic circuit diagramof one embodiment of the invention;

FIG. 2 is a schematic diagram of a second embodiment of the invention;

FIG. 3 is a pulse-time diagram showing the amplitude of the microwavepulses as a function of time for the steps of' the operation of theembodiments of FIGS. 1 and 2;

FIG. 4 is a schematic circuit diagram of a third embodiment of thepresent invention; and

FIG. 5 is a pulse-time diagram of steps of the operation of theembodiment of FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENT Briefly, the present inventionderives very narrow rectangular pulses of the order of nanoseconds inlength from a longer microwave pulse. This invention also shapes thepulse with very fast-rising, leading and lagging edges. This isaccomplished by sequential breakdown in two sections of transmissionline, each of which contains a low pressure ionizable gas acted upon byrelatively intense electromagnetic waves. These sequential breakdownsoccur in the transmission line between the source of electromagneticwave energy and the antenna in the time interval during which the sourceis starting its buildup to a peak amplitude. By adjustment of the delayintervals applied to the trigger pulses applied to the two low pressuredischarges a high power pulse of variable width varying over a very widerange can be produced. The subject matter of this disclosure will bemore readily understood after a brief discussion of operatingcharacteristics of microwaves particularly pertinent to this invention.

The leading edge of a microwave high power pulse, when viewed on ananosecond scale, regardless of the type of tube used as a source,appears as a ramp function whose power level rises from zero to amaximum value equal to the peak amplitude corresponding to the flatportion of the pulse, as indicated particlarly in graph of FIG. 3. Ifthis same pulse is incident on a waveguide section containing alow-pressure gas, which is enclosed by a pair of windows acting solelyas pressure barriers, and if the intensity is sufficiently high, abreakdown of the gas will result. Assuming the insertion loss of a shortsection of line containing the gas to be negligible, then the incidentand transmitted power will rise identically until the gas breaks downinitiating the gas discharge. If then the microwave conductivity of theensuing discharge is sufficiently high the transmitted pulse willrapidly fall toward zero.

Under conditions of high electron density and low collision frequency,relative to the applied signal frequency, the conductivity will belarge; and the discharge acts as an effective barrier to the remainderof the incident pulse. The rising portion of the reflected pulse as wellas the falling edge of the transmitted pulse will be controlled by thetime required for the electron density to rise from a valuecorresponding to negligible RF attenuation to a value of electrondensity corresponding to a relatively high degree of attenuation. Thissituation will become readily apparent from the subsequent description,particularly in reference to the timepulse diagrams explaining theoperation. It will appear from the diagrams that if the length of theramp, that is, the time interval during which the microwave power isincreasing from zero to its maximum peak value, is long compared to thetime interval during which the electron density is rising sharply as aresult of the gas breakdown, the rising portion of the reflected wave upto the maximum source amplitude will occur at a rate far exceeding theoriginal rate of rise of the incident pulse. This is the basicphilosophy of the present invention in which a very steep leading edgeis reflected into the load (antenna) by the waveguide transmissionlines. An analog to this is the electrical induction coil in which theelectrical energy is stored in a magnetic field and which when collapsedby opening the energizing circuit generates a steep high voltage pulse.The apparatus of this invention is a substantial improvement over priordevices.

To proceed further with background discussion, the breakdown of a gasunder the action of intense fields is defined as a transition of theelectron density in the gas from some relatively low initial value to adensity which is orders of magnitude greater at which a steady-statecondition exists. The threshold of breakdown is taken as that value ofapplied field at which the rates of production and loss of electrons areequal. A slight increase of the field beyond this value will cause theelectron density to increase several orders of magnitude to where adynamic equilibrium will exist between the production and loss rates ofelectrons; this avalanche results in complete breakdown.

The rate of increase in the electron density is slow during the rampfunction interval, mentioned previously, when the microwave amplitude isgrowing from zero toward its peak value relative to the decrease in theelectron density when the microwave amplitude is diminishing toward itszero value. The rate of rise of the microwave amplitude as well as therate of decay back to its zero value are functions of the magnitude ofthe applied field, nature and the pressure of the gas, initial densityand the geometry of the area in which the gas discharge takes place.

Microwave breakdown of a low pressure gas requires an initial electrondensity to be present in the discharge region prior to the gasbreakdown. This prevents statistical fluctuations in breakdown energywhich would occur if the natural background density was not sufficientlymasked. This initial density is conventionally provided either by anappropriately located DC discharge or by a radioactive source disposedin the discharge region. These priming devices create a relatively lowdensity, in the neighborhood of 10" to 10 electrons/cm whose spatialdistribution is both nonuniform and highly localized relative to thefield distribution in the waveguide.

With the pressure and the applied field constant for a given geometrythe interval of the microwave amplitude buildup is a function of thespatial distribution of electrons, their distribution of velocities andtheir initial density over the volume of the waveguide where the appliedfield is high. Under conditions where the applied field sees: (a) aninitial density sufficiently high to mask the background density, (b)uniform distribution of electrons in the center half of the waveguide,and (c) a Maxwellian distribution of electron speeds with an averagetemperature of a fraction of electron volt which is equivalent to anisothermal inactive plasma, optimum conditions exist for a jitter-freebreakdown interval with a reproducible breakdown characteristic whenoperation is repeated at high PRF. If the charge density builds upuniformly over the discharge region as in a bulk interaction; theconsequence is a stable and reproducible breakdown with each succeedingpulse.

With initial priming of the region with these electrons, the action ofthe field is to increase the charge density, first in the local regionwhere the initial density is high, and then to depend upon diffusiongradients to initiate a volume breakdown. These diffusion gradientsrequire a finite time to spread the electrons throughout the volume, theinterval being a function of the initial distribution and instabilitiesin the priming sources. Therefore, the charge gradients may fluctuatewith each successive breakdown, the result being in breakdown amplitude.As will appear apparent from the subsequent discussion of the specificembodiments the desired charge and distribution can be approximated byforming a sheet of electrons in the section of the transmission line andthis is done in this instance by the use of triggered PWS switches.

The electron beam creates a plasma, or cloud, of electrons which is thenallowed to decay. In the early afterglow the density, rate of diffusion,and temperature of electrons are high. Late in the afterglow, however,the electrons lose all trace of their ordered motion and rapidlythermalize approaching an isothermal plasma; the electrons then decay bya combination of ambipolar and free diffusion. The electrons lose theirenergy faster than their density decreases. Low-pressure hydrogenplasmas decay only by diffusion and the densities being considered inconnection with this invention are sufficiently high so that thediffusion is primarily ambipolar. By appropriate selection of theparameters it is calculated that the electron temperature decays ordersof magnitude before the number of density decays to 37 percent of itsmaximum value. Therefore, for this condition the electrons cool muchmore rapidly than the density decays and therefore an isothermal plasmain the late afterglow is obtained.

The microwave energy applied to the low pressure gas causes a dischargein the waveguide. The breakdown of the gas is delayed so that the waveis incident on the second low pressure waveguide section during thedelayed afterglow period when the conditions discussed above prevail.This will be clear from the time pulse diagrams which will further bediscussed subsequently. Also the time delay between the initiation ofthe beam to produce the remanent density and the arrival of themicrowave pulse is not critical because the remanent density late in theafterglow period is changing relatively slowly compared to the rise timeof the high power source pulse which is also clearly shown in the pulsetime drafts.

Accordingly, it will be seen that the remanent density provides aninactive plasma suitable to facilitate repeatable breakdowncharacteristics with each applied microwave pulse. In effect, conditionsare favorable to the establishment of high stability for the reflectedleading edge of the microwave pulse. Interpreted in terms of arepetitive output pulse, the peak output power will remain constant, therate of rise of the reflected pulse will repeat identically, and thelocation in time of the initiation of breakdown will not fluctuate thuseliminating leading edge jitter.

The features discussed above will be further delineated in thesubsequent description of speciflc embodiments of apparatus for carryingout the objectives of the present invention as reference is now made tothe drawings.

Referring specifically to the drawings for the illustration of thepreferred embodiments and starting with FIG. 1, a source of microwaveenergy, such as microwave generator 5, is coupled through appropriatewaveguide transmission line 6 through a circulator 7 to a load in theform of an antenna 8. The microwave generator 5 is connected to the No.1 arm of the circulator and the No. 3 arm of the circulator is connectedto the antenna. The No. 2 arm of the circulator 7 is connected to a 3 dBhydrid power divider l2 and a transmission line 11 is connected to theNo. 4 arm of the circulator. The line 11 terminates in an attenuatingload 13. The power divider 12 is connected to a portion of microwaveguide (not shown) which is completely isolated from the transmissionline between the microwave generator and the antenna 8. This portion ofmicrowave guide includes two sections of microwave guide connectedtogether with a small bleed aperture with the outer ends of these twosections closed by pressure windows. The purpose of the bleed aperturebetween the sections is to maintain the same pressure on both of the PWSswitches. The total length of the waveguide portion is such that theswitch PWSl is spaced one-quarter wavelength in the waveguide from theswitch PWS-2.

As will be apparent from the following description the waveguide sectionincluding the two PWS switches shown in FIG. 1 are analogous to a powersink which receives and stores the microwave energy during the intervalwhile the amplitude of the output from the microwave generator 5 isincreasing from zero up to its peak value. With the power divider 12 thetwo PWS switches are connected in parallel and therefore they arecapable of storing twice the amount of microwave power that can bestored if only a single PWS switch was used. Within practical limits thearm No. 2 of circulator 7 could be connected to n number of similarmicrowave sections with appropriate PWS switches in order to furtherenhance the peak of the short pulse output of the system. The reason forthis will be clear when it is seen that both the switches PWS-1 andPWS-2 sees P/2 incident energy, where P is the power output from themicrowave generator 5, resulting in at least twice the incident powerrelative to the case where one PWS switch and no short-slot hybrid isused. Therefore, the useful power output is doubled by the inclusion ofthe hybrid junction feeding the two PWS switches in parallel.

It has previously been said that the PWS switches are preflred so thatthe plasma is in the late afterglow period at the appropriate instant oftime in order to effect the enhancement of the output power pulses inthe operation of the present system. To this end, trigger pulsegenerator 16 supplies control pulses through a delay circuit 17 to themicrowave generator 5. The same output pulses on the output terminal 16aof the trigger pulse generator are also supplied over the conductor 18to a second delay circuit 21. The delay circuit 21 supplies outputpulses simultaneously on conductors 22 and 23 to the switches PWS-1 andPWS-2. This is indicated in the diagram in FIG. 3.

A third delay circuit 24 is also connected to the output terminal 16a ofthe trigger pulse generator and the output of this delay circuit issupplied over conductor 26 to the third switch PWS-3, which is in thewaveguide transmission line 6 connecting the No. 3 arm of the circulator7 and the antenna 18.

The switch PWS-l controls the microwave energy, incident to andreflected from the absorbtion load 27 and likewise the switch PWS-2controls the microwave energy incident to and reflected from theabsorptive load 28. Any microwave energy reflected from No. 3 of thecirculator will be absorbed in the absorptive load 13 connected to No. 4arm of the circulator 7. Also the absorptive load 29 will absorb anyenergy reflected from the 3 dB hybrid junction due to any mismatch inthe transmission line.

From the previous general discussion of the present invention it isbelieved that it would be obvious to those skilled in the art how theinvention works although it may be helpful to add some specific commentsabout the operation. The manner in which the narrow sharp rectangularpulses are derived from the longer pulses is illustrated in the graphsof FIG. 3. The top graph (A) illustrates that each of the switchesPWS-1, PWS-2 and PWS-3, in response to the control pulses, puts out verynarrow rectangular current pulses, indicated at 31.

It is to be understood that the single pulse 31 in graph A, FIG. 3 maybe considered to represent the current pulses in each of the PWSswitches even though the pulse in PWS-3 is slightly delayed after thepulses in PWS-l and PWS-2 which are simultaneous. The time relationbetween the current pulses in the switches and the long microwave pulsesfrom generator 5 is illustrated in the other graphs of FIG. 3.

Controlled stable microwave breakdown of a low pressure gas requires aninitial electron density present in the discharge space prior to thebreakdown. This prevents statistical fluctuations in breakdown energywhich would occur if the natural background density was not sufficientlymasked. As distinguished from prior conventional means for providing thepriming electrons for producing the arc discharge by a radioactivesource or by a DC discharge, in accordance with the present inventionthe PWS switches are relied upon to produce a controlled electrondensity environment approaching the theoretically ideal isothermalplasma. A radioactive electron source or a DC discharge is capable ofproducing only a low density environment of about to 10 electrons percubic centimeter. This ideal condition is obtained when the conditionsare such that the applied field is applied to the gas in an environmentwhere (a) the initial density is sufficiently high to mask thebackground density, (b) the uniform distribution of electrons in thecenter half of the waveguide is present and (c) a Maxwelliandistribution of electron speeds with an average temperature of afraction of an electron volt. Such conditions produce a jitter-freebreakdown interval with a reproducible breakdown characteristic which iscapable of operating at a very high PRF.

The above is accomplished by the proper sequence of the triggering ofthe PWS switches and the microwave generator 5. The switches PWS-1 andPWS-2, when fired, produce a beam current pulse represented by the pulse31 in graph (A) of FIG. 3 and may be capable of creating a plasma ofapproximately 10 to 10 electrons per centimeter.

The significant feature of the present invention over the prior artresides in the feature of relying on the late afterglow of the electronbeams in the switches PWS-l and PWS-2. The delay circuit 17 is soadjusted with respect to the output pulses delivered by the delaycircuit 21 to the PWS switches so that the plasma created by the PWSswitches are allowed to decay. In the early afterglow the density, rateof diffusion, and the temperature of the electrons are high. Late in theafterglow, however, the electrons lose all trace of their ordered motionand rapidly thermalize approaching an isothermal plasma since theelectrons decay by a combination of ambipolar and free diffusion.

The above is graphically illustrated in FIG. 3. As previously mentionedthe leading edge of a microwave pulse is a ramp function P2 as theamplitude of the microwave starts at zero and increases to its peakvalue. Referring specifically to graphs (A), (B) and (C) in FIG. 3 itwill be noted that at some instant of time t1, spaced in time from thecurrent pulse 31 in the PWS switches at t0, the trigger pulse from thedelay circuit 17 triggers the microwave generator 5 when the remanentelectron density is about half its original value at :0 and is decayingat the same time that the amplitude of the microwave is increasing alongthe ramp function illustrated in graph (C) of FIG. 3.

This microwave pulse may be of the order of a fraction of a microsecondup to a full microsecond. The long microwave pulse enters the No. 1 armof the circulator 7 and passes out through the No. 2 arm of thecirculator where it is divided between the two output arms of the powerdivider 12 and through the switch PWS-1 into the absorbing load 27 andthrough the switch PWS-2 into the absorbing load 28. It is to beunderstood that any reflected energy due to a mismatch in the powerdivider goes into the absorbing load 29. The threshold breakdown of thegas is taken as that value of applied field at which the production andloss rate of electrons are equal. This point is represented on the graph(C) at the point PBKDN. T is the formative time lag from the instantthat the microwave generator 5 starts to produce the pulse to thebreakdown threshold.

It will be seen from the above that the microwave pulse is delayed withrespect to the beam current pulse in the switches PWS-1 and PWS-2 sothat the microwave pulse is incident on the low pressure waveguidesection during the late afterglow when the conditions are approachingthe characteristics of an isothermal plasma. The time delay between theinitiation of the electron beam pulse in the PWS switches and thearrival of the microwave pulse is not critical because the remanentdensity late in the afterglow period is changing relatively slowlycompared to the rise time of the high power source pulse. This creates afavorable condition for reflected power leading edge pulse stability.Interpreted in terms of a repetitive output pulse, the peak output powerwill remain constant. The rate of rise of the reflected pulse willrepeat identically, and the location in time of the initiation ofbreakdown will not fluctuate.

A slight increase of the microwave amplitude beyond the point at whichthe production rate and the loss rates of electrons are equal, such asthe breakdown point indicated on the graph (C) of FIG. 3, will cause theelectron density to increase several orders of magnitude to where adynamic equilibrium will exist between the production and loss rate ofelectrons; this avalanche results in complete breakdown. It will beapparent from the graph that the rate of increase in electron density isslow during the interval T relative to the increase in electron densityduring the avalanche interval T If the microwave conductivity becomesvery large during the avalanche interval T T then becomes very smallcompared to the ramp interval T and the amplitude of the microwave pulsewill fall to zero, thus marking the trailing edge and determining themicrowave pulse width. It is when the collision frequency y is equal toor less than the frequency of the microwave energy with conditions ofhigh electron density that the microwave conductivity of the gapsbecomes very large and the gas becomes opaque to the remaining power ofthe residual pulse, indicated by curve E of FIG. 3.

Both the intervals T, and T are functions of the rate of rise of themagnitude of the incident microwave energy, the nature and pressure ofthe gas, the initial density and the geometry. Large electron losses canoccur during the period T, relative to the period T which must becompensated for by an increase in the applied microwave field whichresults in a slight increase in the breakdown power. The dominant energyloss mechanism during the interval T is elastic and in elasticcollisions in the body of gas and not diffusion to the walls. Theintervals T and T differ markedly in that losses during T are affectedprimarily by the geometry while the losses during interval T areaffected by the collision frequency. The maximum energy transfer to theelectron gas is a maximum when the collision frequency equals themicrowave frequency.

To realize the full potential of the present invention the breakdowninterval T must be substantially equal to or greater than the rise timeof the microwave pulse. For a constant remanent density pressure andgeometry, the interval T is a function of the rate of rise of theamplitude of the applied microwave field. In all of these operations itmust be assumed that the collision frequency is less than the microwavefrequency as this is the upper boundary for maximum reflected power.

During interval T the avalanche builds up rapidly and the applied fieldis attenuated. Within the plasma the field is both increased anddecreased simultaneously. It is increased because the wave is traversinga medium of negative permitivity and decreased as a result of powerlosses in the discharge and a change in wavelength. This latter effectresults in a waveguide below cutoff. At the conclusion of the interval Tthe plasma in the PWS switches reaches a state of dynamic equilibrium inwhich the charged density approaches values equal to or greater thanelectrons/em Whether the plasma becomes a good reflector or an absorberof microwaves, however, depends upon the ratio of the collisionfrequency to the microwave frequency. The maximum amplitude of thereflected power does not reach the peak value of the microwave pulsewhen the collision frequency is less than the microwave frequency.

One of the most important variables in the gas discharge is thepressure. It can be shown mathematically that the transfer of energy toa gas is inefficient at low pressures where the collision frequency ismuch less than the frequency of the field, that is, where the fieldmakes many oscillations per electron-molecule collision. Thisinefficient transfer of radio frequency power to the gas is extremelyuseful in delaying the charge buildup and thus lengthening the timeinterval T,.

There are actually two competing processes occurring in the gasdischarges of the PWS switches. High remanent density tends to decreasethe breakdown level and the lower pressure tends to increase thebreakdown power. The latter process is dominant but the limit to whichthe pressure can be decreased is restricted by the dimensions of thecontainer and the maximum ordered amplitude of electron gas. Too low apressure under intense fields will cause electrons to be swept to thewaveguide wall as in a DC discharge. If the secondary emission processesat the wall were negligible, the field required for breakdown wouldrapidly rise. However, if the wall is a good source of secondaryemission, then the walls, not the gas, would play a dominant role in thedischarge. This is undesirable from the viewpoint of stability andjitter since the secondary emission coefficient varies with the historyof the surface and is not constant with time.

Returning again to the operation of the specific embodiment of FIG. 1,when the incident energy enters the power divider 12, the power isevenly divided into the absorptive loads 27 and 28, and each of whichaction takes place as discussed so far in connection with FIG. 3. In thegraph (D) of FIG. 3 the area of the triangular pulse P represents thepower in each of the absorptive loads 27 and 28, respectively. When thepulse power acts on the gas in the discharge areas and the electrondensity builds up to the breakdown point and through the avalancheinterval T the ionized gas discharge becomes opaque to the remainder ofthe incident pulse as indicated in the cross-hatched portion ofgraph(E). v

The energy represented by the cross-hatched area under the curve P ingraph (D) is absorbed by the absorption loads 27 and 28. The reflectedpulses from the switches PWS-l and PWS-2 is combined in the powerdivider l2 and enters the outer end of the NO. 2 arm of the circulator7. The combination of the pulses reflected by these switches isillustrated in graph (E). The combination of these pulses thencirculates to the No. 3 arm of the circulator 7 and the first part ispassed by the switch PWS3, to form the relatively narrow pulse P asrepresented by the graph (F) of FIG. 3. After breakdown of the gas inswitch PWS-3, the remainder of pulse P represented as P, in Graph (G),is reflected by switch PWS-3 through arms NO. 3 and No. 4 of thecirculator 7 and absorbed by absorptive load 13.

The delay circuit 24 is so adjusted that the output pulses on conductor16a of the trigger pulse generator 16 trigger the switch PWS-3 to give abeam current pulse 31' indicated in graph (A) of FIG. 3, similar to thepulse 31 in switches PWS-1 and PWS-2. The switch PWS-3 operates in themanner exactly like that of the switches PWS-1 and PWS-2 previouslydescribed. The time interval between the leading edge of pulse P and theinstant of breakdown of the gas in switch PWS-3 determines the width ofpulse P which is delivered to the antenna 8. The remaining portion ofthe pulse P represented in graph (G) in FIG. 3 is absorbed by theabsorptive load 13 which is connected to the No. 4 arm of the circulator7. Thus, it is seen that from a relatively long original microwave pulsea very narrow radiated pulse P graph (F) of FIG. 3, is derived andsupplied to the antenna 8. The significant feature of this invention isthe fact that the pulse width can be made very narrow and to very closetolerances.

In FIG. 2 the power dividing hybrid junction 45 is in the transmissionline 46 between the microwave generator 5 and the No. 1 arm of thecirculator 7. In this embodiment, the microwave energy in the leadingedges of the microwave pulses is dissipated in the two absorptive loads32 and 33 which are connected, respectively, to switches PWS-390 andPWS-41 which are in turn connected to arms of the power divider 45.

The advantage of the latter arrangement over that of the previousembodiment is that it removes the phasing constraints which reduced theinstantaneous operating bandwidth.

The circulators and the microwave generators in both embodiments can beexactly identical. Furthermore, the antenna 18 may be the same as thatin the previous embodiment. The trigger pulse generator 36, the delaycircuit 37 for the microwave generator 36 can be the same as thecorresponding components in the previous embodiment. Furthermore, therelation between the output trigger pulses on conductor 36a and theoutput pulses of the delay circuit 37 connected to the generator theoutput trigger pulses from the delay circuit 38, which controls switchesPWS 39 and PWS-41; and the output trigger pulses from the delay circuit42, which controls the switch PWS-43, may have the same relation to eachother as the corresponding ones in FIG. 1.

The advantage of the second embodiment over the previous one is itsbetter adaptability of radar operation. In this particular embodimentplease not that the No. 3 arm of the circulator 7 is connected to thereceiver 48 through a suitable transmit-receiver protective device 49.The microwave power reflected from the antenna 18 fires the protectingdevice 49 and is reflected again to be absorbed in the absorptive load51 which is connected to the No. 4 arm of the circulator. Anotheradvantage of the second embodiment of FIG. 2 over that in the previousembodiment is that the switches PWS39 and PWS-41 do not have to bespaced by a quarter wavelength and this removes the phasing constraintwhich reduces the instantaneous operating bandwidth.

A third embodiment of the invention is illustrated in FIG. 4. Whereas inthe first two embodiments the leading edge of the microwave pulse isformed by two simultaneous gas discharges, in this embodiment theleading edge is formed by two successive discharges. It is within thecontemplation of this embodiment that broadly a multiple of PWS switchesmay be used, that is, two or more, to form the leading edge. Thisembodiment is capable of making the short pulse output equal to themaximum power level of the long pulse. As in the previous embodimentsthe lagging edge is formed by a single PWS switch.

Referring specifically to FIG. 4, in a manner similar to that describedin connection with the first embodiment, there is a microwave generator56 connected through transmission line 57 to arm No. 1 of a circulator58. The microwave generator is pulse-modulated by voltage pulsessupplied by a trigger pulse generator 59 on output conductor 59a througha delay circuit 61. These output pulses on the conductor 59a are alsosupplied through a delay circuit 62 to a switch PWS-63 in a section ofwaveguide connecting an absorptive load 64 to the No. 2 arm ofcirculator 58. The output of the delay circuit 63 is also supplied toswitch PWS-66 located in a waveguide section connecting the No. 3 arm ofthe circulator to an absorptive load 67. The No. 4 arm of the circulatorS8 is connected to the No. 1 arm of an isocirculator 71, the No. 2 armof which is connected through switch PWS-72 to antenna 73. The No.3 armof the circulator 71 is terminated in an absorptive load 74.

The operation of the embodiment of FIG. 4 is, in principle, the same asthat in the previous embodiments, except that in the present embodimentthe leading edge of the pulse is formed by two or more successivethyratron plasma switch discharges. The pulse time diagram for theembodiment of FIG. 4 is given in FIG. 5.

As indicated in the pulse-time diagram of FIG. 5, all of the PWSswitches are prefired so that the plasma is in the late afterglow at theinstant that the long microwave pulse is initiated. This is shown in thetop graphs (A), (B) and (C) of FIG. 5. Graph (C) shows the time intervalT for the amplitude of the microwave to increase from zero to fullamplitude, P The incident microwave pulse P from generator 56 istransmitted through the No. 1 arm into the circulator 58 and out the No.2 arm through the switch PWS-63 into the absorptive load 64 until thenet charge in the PWS-63 switch reaches the critical density requiredfor the avalanche operation previously described. This time delayinterval in reaching the critical density corresponding to the amplitudeP is the time interval T, indicated in graph (D). At this point, a rapidbuildup in the electron density takes place in one or two nanoseconds,indicated as formative time lag T graph (E), FIG. 5, making the switchPWS-63 opaque to the remaining portion of the incident microwave pulseafter maximum amplitude P is reached. The remainder of the microwavepulse indicated in graph (E) is circulated to the No. 3 arm of thecirculator 58.

During the formative time lag T before the discharge in the PWS-66switch reaches its critical electron density, the leading edge of themicrowave pulse is absorbed in the absorptive load 67. At the end of theperiod T the avalanche occurs, and the remaining portion of themicrowave pulse, indicated in graph (E) of FIG. 5, is transmitted to theNo. 4 arm of the circulator 58.

It will be seen from the foregoing description that the operation of thesystem is effective to slice ofi the leading edge of a long microwavepulse which has not yet reached full amplitude in one or more slices anddissipates the energy of the slices in absorptive loads and supplies asharply defined full amplitude pulse to a load such as the antenna 73.While the invention is illustrated as applied to a radar system theinvention is not limited to that environment.

Furthermore, the microwave power output pulse supplied to the load, orantenna, is multiplied by two by the 3 dB short-slot hybrid powerdivider and the PWS switches over what it would be if only one PWSswitch was used without the hybrid.

In the illustrative embodiment of the invention, an antenna is shown asbeing the load or device being energized by the microwave pulses. Theterm utilization device is used in the claims as a general term to coveran antenna or any other device performing a useful function in responseto energization by the microwave pulses.

It will be noted from graph (E) of FIG. 5 that the microwave pulse inits full amplitude is transmitted to the circulator 71 and through theswitch PWS-72 to the antenna 73. When the switch PWS-72 is fired by thedelayed trigger pulse, supplied through the delay circuit 62, from thepulse generator 59 the end of the transmitted microwave pulse is markedto give the short pulse P shown in curve (F) of FIG. 5, and theisocirculator 71 routes the remainder of the microwave pulse to the loadabsorptive 74.

I claim an my invention:

l. A microwave pulse modulation system comprising a source of microwaveenergy, a utilization device for receiving energy from said source,microwave transmission line means connecting said source of microwaveenergy and said utilization device, means for pulse modulating saidsource, power dividing means connected to said transmission line meansfor directing microwave energy into a plurality of power absorbing meansand means for abruptly reflecting the microwave energy incident uponsaid power absorbing means to reflect microwave energy into saidtransmission line means.

2. The combination as set forth in claim 1 in which said means forabruptly reflecting the microwave energy are thyratron plasma waveguideswitch means for permitting only selected portions of the incidentmicrowave pulse from said source to reach said absorbing means.

3. The combination as set forth in claim 2 plus further switch means insaid microwave transmission line means for preventing all except aselected portion of the microwave pulses from reaching said utilizationdevice.

4. The combination as set forth in claim 3 in which said further switchmeans is a plasma waveguide switch.

5. The combinatiori as set forth in claim 1 in which said microwavetransmission line means includes a circulator having four arms with itsNo. 1 arm connected to said source of microwave energy, its No. 2 armconnected to said power dividing means and its No. 3 arm connected tosaid utilization device.

6. The combination as set forth in claim 5 in which said No. 3 arm ofsaid circulator is connected to said utilization device through athyratron plasma waveguide switch.

7. The combination as set forth in claim 3, and means responsive to saidpulse modulation means for triggering said microwave source at aselected time interval after the triggering of said thyratron plasmawaveguide switch means.

8. The combination as set forth in claim 1 in which' said transmissionline means includes a circulator, having four arms, said power dividingmeans being connected between said source of microwave energy and saidcirculator.

9. The combination as set forth in claim 7, said' power dividing meansbeing connected between said source and No. 1 arm, the No. 2 arm of saidcirculatorbeing connected to said utilization device and .switch meansin said latter arm for permitting only a selected portion of themicrowave pulse to reach said utilization device.

10. The combination as set forth in claim 1 in which said transmissionline means includes a circulator having four arms, the No. 1 arm beingconnected to said source of microwave energy, the No. 2 and N0. 3 armsbeing connected to respective absorptive loads, and switch means in eachof said No. 2 and No. 3 arms, and the No. 4 arm being connected to saidutilization device.

11. The combination as set forth in claim 10 in which said No. 4 armincludes an isocirculator and a thyratron waveguide switch forpermitting only a selected portion of themicrowave pulses to reach saidutilization device.

1. A microwave pulse modulation system comprising a source of microwaveenergy, a utilization device for receiving energy from said source,microwave transmission line means connecting said source of microwaveenergy and said utilization device, means for pulse modulating saidsource, power dividing means connected to said transmission line meansfor directing microwave energy into a plurality of power absorbing meansand means for abruptly reflecting the microwave energy incident uponsaid power absorbing means to reflect microwave energy into saidtransmission line means.
 2. The combination as set forth in claim 1 inwhich said means for abruptly reflecting the microwave energy arethyratron plasma waveguide switch means for permitting only selectedportions of the incident microwave pulse from said source to reach saidabsorbing means.
 3. The combination as set forth in claim 2 plus furtherswitch means in said microwave transmission line means for preventingall except a selected portion of the microwave pulses from reaching saidutilization device.
 4. The combination as set forth in claim 3 in whichsaid further switch means is a plasma waveguide switch.
 5. Thecombination as set forth in claim 1 in which said microwave transmissionline means includes a circulator having four arms with its No. 1 armconnected to said source of microwave energy, its No. 2 arm connected tosaid power dividing means and its No. 3 arm connected to saidutilization device.
 6. The combination as set forth in claim 5 in whichsaid No. 3 arm of said circulator is connected to said utilizationdevice through a thyratron plasma waveguide switch.
 7. The combinationas set forth in claim 3, and means responsive to said pulse modulationmeans for triggering said microwaVe source at a selected time intervalafter the triggering of said thyratron plasma waveguide switch means. 8.The combination as set forth in claim 1 in which said transmission linemeans includes a circulator, having four arms, said power dividing meansbeing connected between said source of microwave energy and saidcirculator.
 9. The combination as set forth in claim 7, said powerdividing means being connected between said source and No. 1 arm, theNo. 2 arm of said circulator being connected to said utilization deviceand switch means in said latter arm for permitting only a selectedportion of the microwave pulse to reach said utilization device.
 10. Thecombination as set forth in claim 1 in which said transmission linemeans includes a circulator having four arms, the No. 1 arm beingconnected to said source of microwave energy, the No. 2 and No. 3 armsbeing connected to respective absorptive loads, and switch means in eachof said No. 2 and No. 3 arms, and the No. 4 arm being connected to saidutilization device.
 11. The combination as set forth in claim 10 inwhich said No. 4 arm includes an isocirculator and a thyratron waveguideswitch for permitting only a selected portion of themicrowave pulses toreach said utilization device.